With advances in drilling technology, it is currently possible to drill horizontal wellbores deep into hydrocarbon producing reservoirs. There are many advantages to a horizontal well completion including extending contact with a producing formation thereby facilitating drainage and production of the reservoir.
Although horizontal wellbores allow more contact with the producing formation, one encounters some difficulties in well completions associated with horizontal wellbores not commonly dealt with in vertical wells. One area of concern in well completions is the inability to effectively cement voids along the horizontal section during a cementing operation. Effective cementing of the tubing to the wellbore is routinely accomplished in vertical wellbores. However, in horizontal wellbores and severely inclined wellbores, i.e. those having an angle of deviation greater than about 45.degree., cementing is much more difficult. Therefore, the efficiency of zone isolation diminishes considerably.
In oil and gas well completions it is imperative to effectively isolate the oil or gas bearing formation from other zones in the wellbore. This is most frequently accomplished by cementing the production casing in place and then perforating the wellbore in the production horizon to allow hydrocarbon flow into the wellbore. With effective cementing, flow up the outside of the production casing is eliminated and the oil and/or gas producing zone is effectively isolated.
Often a failure of the cementing operation occurs in horizontal wellbores because the density of the cement does not allow sufficient displacement of drilling mud and other residue from the tubing/wellbore annulus, thereby resulting in channeling of cement and improper tubing or pipe/formation bonding. Failure also occurs because formation fines and cuttings remain in the borehole after the drilling operation thereby contaminating the cement. When the cement is contaminated, the cement's bonding and strength is interfered with as the well is completed.
The kind of contaminant remaining in a wellbore depends of course on the operation previously conducted therein. Composition of fluids used in previous operations can vary. Drilling muds, completion fluids, and workover fluids have varying compositions.
Drilling muds are those fluids used in rotary drilling to cool and lubricate the bit, lift cuttings from the borehole, and to control borehole pressure. Drilling muds should have a viscosity, density, and a fluid retention level suitable for the particular drilling application and the formation being drilled. For instance, a drilling mud generally must be sufficiently dense to control the pressure of the well and simultaneously have sufficient plasticity to carry and lift cuttings from the well.
Completion fluids are those fluids used after drilling is complete and during the steps of completion of the well. Completion can include cementing the casing, perforating the casing, setting the tubing the pump, etc.
Workover fluids are those fluids used during remedial work in the well. This can include removing tubing, replacing a pump, cleaning out sand or other deposits, reperforating, etc. Workover also broadly includes steps used in preparing an existing well for secondary or tertiary oil recovery such as polymer additions, micellar flooding, steam injection, etc., or for well stimulation such as acidizing, and fracturing the formation, etc.
If problems involving the composition of the various fluids used in a wellbore weren't enough, there is the added problem of air pockets within a horizontal wellbore which prevent portions of a wellbore from being contacted with a cleaning fluid.
Therefore, what is needed is a method for improving the effectiveness of the cementing operation in a deviated or horizontal wellbore which provides complete removal of contaminants and void spaces in the horizontal section due to incomplete displacement of wellbore materials and the effects of gravity on high density cement.